Hitherto, as catalysts used in the production of methacrylic acid by vapor-phase catalytic oxidation of methacrolein or the like, it is known that those composed of a hetero polyacid and/or a salt thereof are effective, and there are a large number of reports regarding compositions, structures, physical properties, and production processes thereof (for example, Patent Document 1 describes pores of catalysts, Patent Document 2 describes a shaping method in catalyst production, and Patent Document 3 describes a calcination method in catalyst production).
A hetero polyacid-based catalyst used in the production of methacrylic acid by vapor-phase catalytic oxidation of at least one selected from the group consisting of methacrolein, isobutyraldehyde and isobutyric acid with a molecular oxygen-containing gas in the presence of an oxidation catalyst composition has a high moisture absorbability and, once it absorbs moisture, it is known that it is difficult to enhance activity again although the difficultness depends on the degree of moisture absorption. Thus, there is a problem that catalytic performance decreases owing to the moisture absorption during the storage from the production of the catalyst until the use thereof and satisfactory conversion and selectivity of methacrylic acid are not obtained in methacrylic acid production.
Patent Document 4 describes an invention that a catalyst for methacrylic acid production is stored in a container having a moisture permeability of 1.0 g/m2·24 h or less at 25° C. Patent Document 5 describes an invention that air is introduced so that relative humidity in a catalyst layer is controlled to 40% or less to prevent moisture absorption of a catalyst. However, these known technologies are useful as methods for preventing moisture absorption of the catalyst for metharylic acid production but there is a problem that any of steps from the production of the catalyst until the use thereof becomes tedious and complex.
On the other hand, the hetero polyacid-based catalyst used in the production of metharylic acid from at least one selected from the group consisting, of methacrolein, isobutyraldehyde and isobutyric acid is indeed interior in life to the oxidation catalyst for producing acrylic acid from acrolein.
Patent Document 6 discloses a process for producing a catalyst for methacrylic acid production by mixing a homogeneous solution containing a fourth ingredient X such as molybdenum, vanadium, phosphorus and antimony with a homogeneous solution containing the other catalyst ingredients such as ammonia water and cesium and drying the mixed solution. Thereby, it is said that solubility of the fourth ingredient X (especially antimony) is enhanced and a catalyst exhibiting excellent reproducibility and stability of catalytic performance and having a long life is obtained. Moreover, Patent Document 7 discloses a production process of an oxidation catalyst wherein, with regard to a solution obtained by dissolving or suspending all catalyst raw materials in water, the content of an ammonium radical is controlled to the range of 17 to 100 mol relative to 12 atoms of molybdenum and pH thereof is controlled to the range of 6.5 to 13. The control of the pH is performed through addition of nitric acid, ammonia waters or the like.
However, catalysts produced using such conventional method for mixing catalyst raw materials or method for controlling pH are not always sufficient as industrial catalysts particularly in view of life and it is desired to further improve catalytic performance.